The desulfurization process employed for molten iron is significantly different in foundries from that utilized in integrated steel mills. The scale of the two processes has led to materially different approaches to the desulfurization thereof by the two industries.
Whereas integrated steel mills, for the most part, batch desulfurize about 50-300 ton batches of hot iron by injecting 400 mesh powders 6-10 feet below the iron surface using a lance and a carrier gas, foundry desulfurization normally entails the surface addition of from about 8 to about 80 mesh powders to dwell units of from about 1-10 ton units of hot iron. Foundry desulfurization may be performed in batch, semi-continuous or continuous fashion.
The injected powder in the integrated steel mill desulfurization process typically contains a reducing gas-generating additive which may also assist in the desulfurization of the metal. An example of such an additive is magnesium. The gas-generating additive in this type of process performs the necessary stirring function to enable homogeneous desulfurization in the large capacity integrated mill desulfurizing vessel. Because surface addition of the desulfurizing agent is used in foundry desulfurization of iron, a gas-generating additive is not necessary and is very infrequently used for stirring. In the absence of the gas-generated additive, the stirring function is performed by an extraneous stirring means such as porous plugs inserted into the bottom of the dwell unit through which nitrogen gas is bubbled or a mechanical paddle type of stirring mechanism.
A concise dissertation of the various procedures for desulfurizing iron can be found in Ductile Iron; Molten Metal Processing; Chap. 3; 2nd Ed. Published by the American Foundrymen's Society; Des Plains, Ill; 1986. Examples of procedures employed by the integrated mills are set forth in U.S. Pat. Nos. 3,885,956 and 3,929,464, hereby incorporated herein by reference.
The injection technology employed by the integrated mills has generally not been adopted for use by the foundries due to the high capital cost of the injection system relative to the pneumatic or gravity feed surface addition systems used by the foundries. Also insufficient ladle depth in the foundry dwell units does not make powder injection practical or efficient. Thus, surface addition of desulfurization reagents has been found to be the only viable alternative for foundries.
The requirements for foundry desulfurization reagents have been established over the years through economic, safety and environmental necessities. These reagents must:
1. be capable of effectively and efficiently removing sulfur from iron containing 0.015-0.1% or more sulfur to 0.010% or lower. PA1 2. be sized within a narrow range of about 8 to 80 mesh, preferably about 10 to 35 mesh, and not contain excessive amounts of fines (to prevent eye and skin irritation) or coarse material (to insure good reagent efficiency and low residual calcuim carbide in the slag.)
These requirements have been reasonably satisfied by available commercial reagents in the past, however, an ever increasing environmental awareness has placed even more stringent requirements on foundries.
One of the most important environmental effects has been caused by residual calcium carbide in the slag resulting from the desulfurization process. When the slag contains large amounts of this material, disposal thereof is further complicated because calcium carbide has been determined to be a safety hazard.
The cause of residual calcium carbide in the slag has been determined to be the presence of large particles in the charged reagent which are not completely used up during the desulfurization reaction. The normal chemical reaction which occurs during the desulfurization of the molten iron is both the reaction of the calcium carbide with the sulfur in the metal to form calcium sulfide and carbon and the reaction of the calcium carbide with oxygen to form calcium oxide and carbon dioxide or carbon monoxide gas. All of these reaction products can be dealt with by the foundries by existing methods.
The problem arises when the calcium carbide of the larger particle size becomes coated with the calcium sulfide or calcium oxide reactants and is not completely reacted. This unreacted calcium carbide finds its way into the resultant slag and must be removed before disposal of the slag can be accomplished.
Attempts to overcome this residual calcium carbide slag problem have included utilizing very fine particles of calcium carbide as a charge so as to assure complete conversion thereof. However, the very essence of the foundry surface addition system prevents this solution because convection from the molten iron process per se carries the small unreacted particles away as does the suction from the fans over the dwell units required to dissipate fumes, dust etc. Thus, the use of fine particles results in a further safety hazard at the foundry and a large loss of material into the baghouse dust collection system.
Thus, the useful particle size of the calcuim carbide is virtually governed by the system employed and foundries have faced the residual calcium carbide problem by treating the slag through a controlled water addition to generate acetylene and thus reduce the carbide content of the slag. This extraneous water treatment, however, puts an additional burden on the foundry and creates a further expense not to mention the safety hazards of such, odor and dust generation and the water purification requirements which result. Thus, the existence of a system which eliminates or materially reduces the content of residual calcium carbide in the slag of foundry desulfurization would solve a continuing problem in the iron industry.